1. **Preheat the Oven:**

– Preheat your oven to 375°F (190°C). Line a baking dish with parchment paper or lightly grease it.

2. **Prepare the Zucchini:**

– Wash the zucchini and slice them in half lengthwise.

– Using a spoon, carefully scoop out the flesh of each zucchini half, leaving about a 1/4-inch border to create “boats.” Reserve the scooped-out flesh and chop it finely.

3. **Cook the Filling:**

– In a large skillet over medium heat, heat the olive oil.

– Add the Italian sausage, breaking it up with a spoon, and cook until browned.

– Add the diced onion and cook until translucent.

– Stir in the minced garlic and cook for an additional minute until fragrant.

– Add the chopped zucchini flesh, marinara sauce, Italian seasoning, and red pepper flakes (if using). Cook for 5-7 minutes, allowing the mixture to thicken slightly. Season with salt and pepper to taste.

4. **Stuff the Zucchini Boats:**

– Arrange the zucchini halves in the prepared baking dish.

– Spoon the sausage mixture evenly into each zucchini boat.

– Sprinkle the shredded mozzarella and grated Parmesan cheese over the top of each filled zucchini.

5. **Bake:**

– Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.

– Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.

6. **Serve:**

– Remove from the oven and let the stuffed zucchini cool for a few minutes.

– Garnish with fresh basil leaves if desired, and serve warm.